1. Experiment with using the different grib_dump options
(-O,  -a and –t).  Inspect the GRIB message in the files file1.grib1 and file1.grib2 and identify:


To view the keys available for GRIB1 and GRIB2 have a look at the parameter database at http://apps.ecmwf.int/codes/grib/

To use grib_dump to inspect the contents of file1.grib1:

% grib_dump file1.grib1

By using grib_dump on its own and with the various options, (-O, -a, etc), you should find:

  • the message is encoded using GRIB edition 1 (editionNumber=1);
  • it contains the 2 metre temperature field (shortName=2t, marsParam=167.128, paramId=167);
  • the dataDate= 20190201, the dataTime=00 and the forecast step=12;
  • the data is on an octahedral reduced Gaussian grid (typeOfGrid=reduced_gg) with N=640 (Nj=1280) and gridName=0640.

Similarly for the file file2.grib2

% grib_dump file1.grib2

By using grib_dump on its own and with various options, (-O, -a etc), you should find:

  • the message is encoded using GRIB edition 2 - editionNumber=2;
  • it contains the temperature field on a specified height of 2 m above ground (shortName=2t, paramId = 167, discipline = 0, parameterCategory = 0, parameterNumber = 0, typeOfFirstFixedSurface = 103);
  • the dataDate=20190201, the dataTime=00 and the forecast step=12;
  • the data is on an octahedral reduced Gaussian grid (typeOfGrid=reduced_gg) with N=640 (Nj=1280) and gridName=0640.


To quickly find the maximum, minimum and average values of the field, look for the computed keys max, min, and average.  

For both files you should find:

  • maximum = 315.501
  • minimum = 222.528
  • average = 287.703


2. Use grib_ls to print the centre, dataDate, stepRange, levelType, shortName and paramId for msl.grib1 and msl.grib2 and order by ascending stepRange.


To list the GRIB messages in msl.grib1, use:

%  grib_ls msl.grib1
edition      centre       typeOfLevel  level        dataDate     stepRange    dataType     shortName    packingType  gridType
1            ecmf         surface      0            20190201     6            cf           msl          grid_simple  regular_gg
1            ecmf         surface      0            20190201     12           cf           msl          grid_simple  regular_gg
1            ecmf         surface      0            20190201     18           cf           msl          grid_simple  regular_gg
1            ecmf         surface      0            20190201     24           cf           msl          grid_simple  regular_gg
4 of 4 grib messages in msl.grib1

The file contains mean sea-level pressure (shortName=msl) from the ECMWF ENS control forecast (dataType=cf) for 6-hourly steps from 6 to 24 hours for dataDate=20190201.  The messages are encoded in GRIB edition 1.

To list the GRIB messages in msl.grib2

%  grib_ls msl.grib2
edition      centre       date         dataType     gridType     stepRange    typeOfLevel  level        shortName    packingType
2            ecmf         20190201     cf           regular_gg   6            meanSea      0            msl          grid_simple
2            ecmf         20190201     cf           regular_gg   12           meanSea      0            msl          grid_simple
2            ecmf         20190201     cf           regular_gg   18           meanSea      0            msl          grid_simple
2            ecmf         20190201     cf           regular_gg   24           meanSea      0            msl          grid_simple
4 of 4 grib messages in msl.grib2


The file also contains mean sea-level pressure (shortName=msl) from the ECMWF ENS control forecast (dataType=cf) for 6-hourly steps from 6 to 24 hours for dataDate=20190201.  In this file the messages are encoded in GRIB edition 2.

The lists show the default keys printed by grib_ls for GRIB 1 and GRIB 2. The same keys are listed but the order in which they appear differs between GRIB edition 1 and GRIB edition 2.

Using the "key:i" for centre and levelType prints these keys as their integer values rather than strings:

% grib_ls -p centre:i,dataDate,stepRange,levelType:i,shortName,paramId msl.grib1
centre       dataDate     stepRange    levelType   shortName    paramId
98           20190201     6            1            msl          151
98           20190201     12           1            msl          151
98           20190201     18           1            msl          151
98           20190201     24           1            msl          151
4 of 4 grib messages in msl.grib1

Similarly for msl.grib2:

% grib_ls -p centre:i,dataDate,stepRange,levelType:i,shortName,paramId msl.grib2
centre       dataDate     stepRange    levelType  shortName    paramId
98           20190201     6            101          msl          151
98           20190201     12           101          msl          151
98           20190201     18           101          msl          151
98           20190201     24           101          msl          151
4 of 4 grib messages in msl.grib2


3. Find the value of the MSLP at the grid point nearest to ECMWF (Lat 51.42°N, Lon 0.95° W) at each forecast step


To list the nearest points to ECMWF (Lat 51.42 N, Lon 0.95 W) use

% grib_ls -l 51.42,-0.95 msl.grib1
   …

4 of 4 messages in msl.grib1

4 of 4 total messages in 1 files
Input Point: latitude=51.42  longitude=-0.95
Grid Point chosen #2 index=156197 latitude=51.38 longitude=359.03 distance=4.79 (Km)
Other grid Points
- 1 - index=156198 latitude=51.38 longitude=359.35 distance=21.64 (Km)
- 2 - index=156197 latitude=51.38 longitude=359.03 distance=4.79 (Km)
- 3 - index=155082 latitude=51.52 longitude=359.35 distance=23.65 (Km)
- 4 - index=155081 latitude=51.52 longitude=359.03 distance=11.09 (Km)


Note we specify the longitude of 0.95 W with -0.95 !

The nearest grid point is at latitude=51.38 longitude=359.03 (=0.97 W) and is 4.79 km from ECMWF.

To output only the forecast step and MSLP value at the nearest grid point use:

% grib_ls -l 51.42,-0.95,1 -p stepRange msl.grib1

msl.grib1
stepRange    value
6           98263.5
12          98621.1
18          99085.2
24          99593.2
4 of 4 messages in msl.grib1

4 of 4 total messages in 1 files
Input Point: latitude=51.42  longitude=-0.95
Grid Point chosen #2 index=156197 latitude=51.38 longitude=359.03 distance=4.79 (Km)
Other grid Points
- 1 - index=156198 latitude=51.38 longitude=359.35 distance=21.64 (Km)
- 2 - index=156197 latitude=51.38 longitude=359.03 distance=4.79 (Km)
- 3 - index=155082 latitude=51.52 longitude=359.35 distance=23.65 (Km)
- 4 - index=155081 latitude=51.52 longitude=359.03 distance=11.09 (Km)

   stepRange    value
   0           102527
   6           102365
   12          102456
   18          102289
   24          101862
   5 of 5 grib messages in msl.grib1

   Input Point: latitude=51.42  longitude=-0.95
   Grid Point chosen #3 index=17199 latitude=51.62 longitude=359.10 distance=22.57 (Km)
   Other grid Points
   - 1 - index=17599 latitude=50.72 longitude=359.10 distance=77.54 (Km)
   - 2 - index=17598 latitude=50.72 longitude=358.20 distance=97.59 (Km)
   - 3 - index=17199 latitude=51.62 longitude=359.10 distance=22.57 (Km)
   - 4 - index=17198 latitude=51.62 longitude=358.20 distance=62.87 (Km)
 
Note it is necessary to specify MODE=1 in order to print the value of the nearest grid point only.  

To output the values at the four grid points nearest to ECMWF, use MODE=4 which is the default:

% grib_ls -l 51.42,-0.95 -p stepRange msl.grib1

msl.grib1
stepRange          value1  value2  value3  value4
6           98236.5 98263.5 98293.5 98319.2
12          98596.1 98621.1 98644.4 98666.1
18          99043 99085.2 99074.5 99116
24          99547 99593.2 99561.5 99612.2
4 of 4 messages in msl.grib1

4 of 4 total messages in 1 files
Input Point: latitude=51.42  longitude=-0.95
Grid Point chosen #2 index=156197 latitude=51.38 longitude=359.03 distance=4.79 (Km)
Other grid Points
- 1 - index=156198 latitude=51.38 longitude=359.35 distance=21.64 (Km)
- 2 - index=156197 latitude=51.38 longitude=359.03 distance=4.79 (Km)
- 3 - index=155082 latitude=51.52 longitude=359.35 distance=23.65 (Km)
- 4 - index=155081 latitude=51.52 longitude=359.03 distance=11.09 (Km)

To specify the land-sea mask, use:

% grib_ls -l 51.42,-0.95,1,lsm.grib1 -p stepRange msl.grib1

msl.grib1
stepRange    value
6           98263.5
12          98621.1
18          99085.2
24          99593.2
4 of 4 messages in msl.grib1

4 of 4 total messages in 1 files
Input Point: latitude=51.42  longitude=-0.95
Grid Point chosen #2 index=156197 latitude=51.38 longitude=359.03 distance=4.79 (Km)
Mask values:
- 1 - index=156198 latitude=51.38 longitude=359.35 distance=21.64 (Km) value=1.00
- 2 - index=156197 latitude=51.38 longitude=359.03 distance=4.79 (Km) value=0.99
- 3 - index=155082 latitude=51.52 longitude=359.35 distance=23.65 (Km) value=0.99
- 4 - index=155081 latitude=51.52 longitude=359.03 distance=11.09 (Km) value=0.99

Here the mask value at the end of the line showing the four nearest grid points indicates which are land points: Land points have mask ≥ 0.5; sea points have mask<0.5.

This works in the same way for msl.grib2.  Note that you can pass the land-sea mask file as either GRIB edition 1 or GRIB edition 2.